The technique of UWB communication in pulsed mode consists in transmitting pulses of short duration (of the order of one nanosecond). The pulsed signals concerned have a bandwidth of several gigahertz. Information may be coded via the level or the amplitude of these pulses (for example by pulse amplitude modulation (PAM)) and/or via the position in time of these pulses (pulse position modulation (PPM)).
A signal v(t) of the type considered in the context of the impulse mode UWB communication technique may generally be expressed in the time domain as the product of a sinusoid portion with center frequency fc by a temporal window w(t), that is to say, in analytical form:v(t)=sin(2πfc×t)×w(t)
The temporal windowing governs the shape of the spectrum and a rectangular, triangular or Gaussian gate may be used for the window w(t). In the latter case:
      w    ⁡          (      t      )        =      exp    ⁡          [              -                              (                          t              τ                        )                    2                    ]      where τ characterizes the temporal length of the pulse.
The center frequency fc is typically made equal to 4.1 GHz and the pulse width τ typically has the value 342 ps, which corresponds to a −10 dB spectral occupancy of 2 GHz. The graphs in FIGS. 1A and 1B show the form of such a signal in the time domain and in the frequency domain, respectively.
In FIG. 1A, time t (in nanoseconds) is plotted on the abscissa axis and the curve represents the level of the signal v(t) in volts. In FIG. 1B, frequency (in GHz) is plotted on the abscissa axis and the spectral power density (DSP) (in dBm/MHz) is plotted on the ordinate axis. The signal is transmitted with a frequency transmission mask as authorized by the FCC (Federal Communications Commission). The signal is represented in solid lines and the FCC mask is represented in dashed lines.
To transmit information, as mentioned above, several types of modulation may be used, in particular modulation by level or modulation by position in time. For example, there may be used a form of modulation by level of the all or nothing modulation type (OOK, On-Off Keying; for simplicity, in the remainder of the description, the expression “OOK modulation” is used) or a form of modulation by position in time of the N-pulse position modulation type (N-PPM, N representing the number of nominal positions; in the remainder of the description, for simplicity, the expressions “PPM modulation” and “N-PPM modulation” are used).
The three timing diagrams in FIG. 2 give a diagrammatic temporal representation of these two types of modulation.
In OOK modulation (see the center timing diagram in the drawing), a pulse is sent when the logic level has the value 1 and nothing is sent when it has the value 0. The pulse repetition interval (PRI in the drawing) here corresponds to the binary timing. This interval may be divided into two impulse position intervals (IPI in the drawing).
In PPM modulation (see the bottom timing diagram in the drawing: the case of 2-PPM modulation is illustrated by way of nonlimiting example), if the information has the value 1, the pulse is sent in the first interval, and if the information has the value 0, the pulse is sent in the second interval.
The UWB telecommunication sector has gained a new relevance in recent years. In particular, solutions have been proposed for providing high bit rate communications, but these architectures cannot address the new expectations in this field in terms of low consumption and low cost.
A solution of the “rake receiver” type is known from published PCT application No. WO-A-01 76086. The receiver described in that document uses an analog mixer to establish a correlation between the received signal and an elementary pulse pattern generated in the receiver. This solution concentrates on a finite number of collected paths. Each finger or path on which the receiver concentrates requires an active mixer, increasing the complexity and the consumption of the device.
Another solution consists in sampling the UWB signal and establishing a correlation with a digital reference pattern, in order to demodulate the signal. This correlation pattern takes into account the complete impulse response of the channel, in contrast to the previous solution. Nevertheless, this solution is relatively complex to implement: because of the high bandwidth and the high center frequency, fast sampling is required in order to comply with the Nyquist criterion, necessitating a high frequency synthesis and producing a voluminous stream of digital data. All these operations, which are costly in energy, are effected to the detriment of the overall consumption of the receiver.
Solutions are emerging for reducing the complexity of the reception architecture, but they still have major drawbacks:
A first of these solutions is based on detecting pulse peaks. There are two techniques for implementing this solution.
The first technique corresponds to a comparator type threshold detector. This solution necessitates high amplification of the received signals because the sensitivity of such detectors is generally relatively low. Consumption is then governed by the stages with gain.
A second technique known from U.S. Pat. No. 5,901,172 corresponds to the use of a tunnel-effect diode: the diode is biased to the peak voltage in the vicinity of the negative slope zone of instability. If the level of the signal is sufficient, it triggers the tunnel effect in the diode and the operating point switches into the “valley” zone of the characteristic of the diode, thus enabling the presence of a pulse to be detected. A drawback of this solution is the drifting of the bias point of the diode as a function of temperature and power supply fluctuations, which degrades the sensitivity of the receiver. Solutions exist that attempt to remove this drawback, for example by placing signal attenuators for adjusting the bias point, but these compensation solutions are hardly effective in terms of energy consumption.
Another solution consists in working with energy detection architectures. These architectures lump together all of the energy recovered in the propagation channel. This operation necessitates the rectification of the UWB signal before integrating the result over an integration window corresponding to the spreading of the channel. However, the very low levels of the received signals (of the order of magnitude of a few tens of microvolts) generally mean that this operation cannot be effected without proceeding beforehand to significant amplification. This amplification requires a high consumption, which limits the benefit of such a solution. Moreover, the fact of lumping together all of the energy rules out exploiting the excellent temporal resolution offered by UWB.
Thus the prior art cited above does not address the need for an architecture and a process for receiving ultrawideband signals that are simple to implement, enable the development of receivers having a low consumption, and can respond to location and communication requirements.
The RF reception architecture based on super-regenerative devices was described for the first time in 1922 by E. H. ARMSTRONG in a paper entitled “Some recent developments of regenerative circuits”, in Proc. IRE, vol. 10, August 1922, pages 244 to 260. The principle of the super-regenerator is as follows: at the heart of the device there is an oscillator placed at the limit of instability. When the signal arrives, the additional energy it contributes triggers the oscillator, which begins to oscillate. If there is only noise at the input of the device, starting is effected much more slowly, making it possible to distinguish between the situation in which a signal is present and the situation in which there is only noise. The general theory of using this device for the reception of RF signals is described in a specialist work by J. R. WHITEHEAD entitled “Super-Regenerative Receivers” published by Cambridge University Press in 1950.
This architecture has the advantage of providing quasi-infinite amplification at low power consumption. It found applications in the 1950s in the field of pulsed radar, where the level of the reflected signals to be processed was very low. On this subject see U.S. Pat. No. 3,329,952 applied for in 1957 and granted in 1967.
The implementation options of that time render the production of such low-voltage integrated micro-electronic technology devices difficult at present, however.
Moreover, in the applications considered at the time, the signals propagated in free space, whereas the environments currently envisaged in UWB applications are very different and very varied, and necessitate a particular adaptation of the reception architecture and of the corresponding reception process.
The super-regenerator has been used in recent years to demodulate a narrowband signal rather than an ultrawideband signal, with or without spectrum spreading. On this subject see published PCT application No. WO-A-03 009482. An advantage of this solution lies in its low cost of implementation and its low consumption, justifying its use in remote control type applications, for example. In this solution, demodulation is effected by direct conversion of the RF signal into the baseband. The current trend in narrowband applications is to use high-Q resonators of the surface acoustic wave (SAW) type to improve the selectivity of the super-regenerative architecture. Such a device is described in U.S. Pat. No. 4,749,964.
Despite all this, when applied to narrowband communications the super-regenerator has a major drawback, namely a bandwidth that is too large compared to the band of the modulated signal. This makes communication vulnerable because it is sensitive to nearby sources of interference and to accumulated in-band noise.
It is for this reason that frequency-transposition-based homodyne or heterodyne architectures have been preferred when the major constraint is not consumption.
On the other hand, in the UWB context, the drawback cited above may in contrast prove to be a beneficial advantage, in that, as indicated in the introduction, the band of the signal occupies several GHz. A receiver with a high bandwidth is therefore required to obtain good detection performance.
Furthermore, because of the inherent operation of the super-regenerator, the circuit does not recover all of the energy that is transmitted on a sinusoidal carrier, for example; it is sensitive to only a portion of that energy, corresponding to the phase in which the oscillator is at the limit of instability. Thus the super-regenerator inherently samples the input signal. This property is particularly well adapted to UWB signals in that the energy is concentrated over the time period in which the UWB pulse is transmitted. Finally, in the narrowband field, unlike UWB, a tuning difference between the resonant frequency of the super-regenerator and the carrier frequency of the signal degrades the sensitivity of the device. Production constraints therefore apply to narrowband devices that are not relevant when a spread spectrum is used for excitation.
Accordingly, and contradicting perceived wisdom in the art, despite the frequency selectivity of the oscillatory device of a super-regenerator, the latter is, at constant average power, more efficient in a UWB system than in a narrowband system.